NON-CRISS-CROSS INHERITANCE. 27! 



triploid for X. It is clear that from time to time a three X female 

 survives. She has gray wings and body color like the father, be- 

 cause two yellow genes are recessive to one wild type gene. In a 

 culture where the father was bar, the X-triploids had eyes less bar 

 than in a heterozygous female, yet showing the character bar ; this 

 is known to be the appearance of bar when carried in one chromo- 

 some in the presence of two other chromosomes, both carrying the 

 gene for wild type. X-triploids have not been observed in all of 

 the cultures, and the percentage of them when present varies very 

 much. In one culture their number was for a time about equal to 

 the number of yellow females. All of many attempts to breed 

 them have failed. 



OCCASIONAL BREAKING APART OF THE DOUBLE CHROMOSOME. 



No exception to the non-criss-cross inheritance was observed 

 until the F 4 generation, when a single yellow male appeared in a 

 line in which all the other males were wild type. The yellow 

 chromosomes of the mother had in this case presumably broken 

 apart and the usual type of sex-linked inheritance from mother to 

 son had been restored. The fly bred like an ordinary yellow male. 

 At about the same time, in each of two related cultures of double 

 yellow females by cross-veinless, cut, forked males, there appeared 

 a wild type female not having the characteristics of an X-triploid. 

 Both of these flies were fertile and both bred like females having 

 one yellow-bearing X-chromosome, and one X-chromosome bearing 

 the characters of the males of the cultures I.e., having the differ- 

 entials for cross-veinless, cut and forked. One of these females 

 was bred to a brother and produced females of two classes (wild 

 type and cross-veinless, cut, forked) and yellow sons, and cross- 

 veinless, cut, forked sons and the cross-over classes to be expected. 

 The other wild type female was bred to a 7ple male and also pro- 

 duced yellow sons and cross-veinless, cut, forked sons, and females 

 and cross-overs of the classes to be expected. These two females, 

 like the yellow male of the other culture, had received from the 

 mother a single yellow chromosome, broken apart from its mate. 

 Since these flies occurred there have appeared in cultures of double 

 yellow females six other yellow males and two wild type, not X- 

 triploid, females. All the males have been bred and all but one 



